The overall goal of these studies is to define the receptor-specific actions of endothelin (ET) in an effort to better understand the renal mechanisms of salt-dependent hypertension. There is considerable evidence that ET contributes to hypertension in salt-dependent models such as the DOCA-salt treated rat. The basis of our investigations is the general hypothesis that the balance between ETA and ETB receptor actions in the kidney is tilted in the direction of pro-hypertensive pathways in salt-dependent hypertension. When normal rats are challenged with a high salt diet, the ETB receptor within the renal medulla stimulates NO synthase (NOS) and cytochrome P450 4A (CYP4A) to produce NO and 20-HETE, respectfully. These mediators serve to prevent elevations in arterial pressure by inhibiting tubular sodium reabsorption either directly or by influencing the renal microcirculation. In salt-dependent hypertension, we propose that the balance of ET actions shifts away from the blood pressure lowering effects of the ETB receptor. The resulting stimulation of ETA receptors increases intrarenal oxidative stress through NADPH oxidase-mediated superoxide production. This, in turn, reduces NOS activity and compromises the ability of the kidney to regulate renal hemodynamics and sodium excretion. We also propose that generation of hydrogen peroxide contributes to the derangements associated with ETA-dependent oxidative stress. The net effect of the increased oxidative stress, a lack of increased NOS and CYP4A activity, contributes to a reduced ability to excrete sodium, and thus, elevations in arterial pressure. Experiments will use a variety of animal models including DOCA-salt hypertensive rats and mice and chronic ETB receptor blockade, to determine ET receptor specific mechanisms in the control of arterial pressure and kidney function. We will use both pharmacological tools and genetically modified animals to elucidate the pathways described in the above scheme. The activity and expression of enzymes and mediators in key pathways will be determined in renal medullary tissue in normotensive and hypertensive animals. Aim 1: To test the hypothesis that ETA stimulation of NADPH oxidase contributes to reduced NOS activity in DOCA-salt hypertension. Aim 2: To test the hypothesis that hydrogen peroxide contributes to hypertension in DOCA-salt hypertensive rats. Aim 3: To test the hypothesis that up-regulation of CYP4A can reduce salt-dependent hypertension associated with reduced ETB receptor function.